Electroplating on maraging steels



United States Patent 3,338,803 ELECTROPLATING 0N MARAGING STEELS George A. Di Bari, Haverstraw, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware N0 Drawing. Filed Oct. 12, 1964, Ser. No. 403,342 4 Claims. (Cl. 204-34) The present invention is directed to the electroplating of maraging steels and, more particularly, to a method for providing sound adherent electrodeposits of metals such as nickel and chromium upon articles made of solution annealed or hardened maraging steel.

The advent of the maraging steels upon the metallurgical scene has made available to the designer a new family of materials which affords a very high strength combined with a substantial degree of toughness. The high strength developed in these new materials enables the designer of highly stressed articles to provide articles which are highly stressed in service and which have an advantageous strength/weight ratio. The fabrication of highly stressed articles at a saving in weight as compared to prior steels and other metals of lower strength has thus been facilitated. The possibilities made available by the maraging steels have led to the use thereof in critical applications wherein the saving in weight can be utilized commercially. An example of such an application is an aircraft landing gear strut. In providing such articles, it has been found desirable to shot peen the surface of the hardened finished article in order to improve the fatigue resistance of the article. In many instances a dramatic improvement in fatigue resistance as revealed by standard testing methods, e.g., on the order of a ten fold improvement, has been found to flow from the use of shot peening as applied to hardened articles made of maraging steel. It has been found desirable in certain critical applications to provide an electroplated coating of a metal such as nickel, cadmium, chromium or other commonly electrodeposited metals or combinations thereof on the surface of the finished hardened maraging steel article in order to contribute further advantages from the standpoint of corrosion resistance and abrasion resistance to the finished maraging steel article. One very effective coating system involves a sound, adherent nickel basis deposit on the order of one to two mils in thickness with an outer chromium deposit up to eight mils thick applied thereto as this coating system combines the abrasion resistance of chromium with the corrosion resistance of nickel. It has been found, however, that difiiculties have been encountered in securing good adherence of electroplated metal coatings on the surface of hardened maraging steel articles particularly when the articles have been shot peened. This difiiculty has been indicated, for example, in that baking of electroplated hardened maraging steel which had been electroplated with, for example, nickel, by prior methods resulted in blistering of the electroplated coating with the consequence that the finished article had to be rejected. The blistering of the coatings upon baking of the coated article appears to occur randomly and has also been accompanied by cracking and flaking of the coatings.

I have now developed a method for treating solution annealed or hardened maraging steel surfaces which enables the production of adherent electrodeposited metal coatings thereon.

It is an object of the present invention to provide a method for producing adherent metallic electrodeposits upon the surface of solution annealed or hardened maraging steels.

It is a further object of the invention to provide a means for treating the shot peened surface of a hardened maraging steel article to permit the electrodeposition thereon of a sound adherent electrodeposited metal coating which does not blister when the electrodeposited article is subjected to baking.

Other objects and advantages of the invention will become apparent from the following description.

Generally speaking, the present invention comprises the treatment of a solution annealed or hardened maraging steel article to provide an adherent electrodeposited coating thereon which comprises, in sequence, subjecting the steel surface to an anodic treatment for at least about 1 minute up to about 5 minutes, e.g., about 2 minutes, at a current density of about 200 to about 250 amperes per square foot (a.s.f.) in a solution containing about 25% to about 37.5% by weight (about 250 to about 375 grams per liter) of sulfuric acid (H immersing the article for a brief period in an aqueous solution or bath containing about 10 to about 500 grams per liter (g.p.l.) chromic acid (CrO and about 1 to about g.p.l. of sulfuric acid and then immersing the steel article in an aqueous acid nickel chloride bath containing about 200 to about 300 g.p.l. of nickel chloride (NiCl -6H O) and about 30 tov about 40 g.p.l. of hydrochloric acid for about 1 minute to about 6 minutes and then electrodepositing nickel upon said steel surface from said bath at a current density of about 20 to about 50 a.s.f. from about 1 to about 6 minutes. The steel article having a thin nickel strike coating of about 0.015 to about 0.1 mil in thickness is then in a condition permitting the deposition thereon of an adherent electrodeposited coating of a metal from the group consisting of nickel, chromium and cadmium and is transferred to a standard plating bath for plating such a metal thereon. More advantageously, the chromic acid-sulfuric acid immersion bath contains about 40 to about 60 g.p.l. of chromic acid (CrO and about 35 to 55 g.p.l. of sulfuric acid. The immersion treatment is quite short, e.g., about 1 to about 3 minutes. A 1 minute dip is usually sufiicient.

It is to be understood that the final plating for production of a nickel coating may be conducted in any of the commonly usednickel baths, e.g., the all-sulfate bath, the sulfate-chloride bath, the all-chloride bath, the sulfamate bath, the fluoborate bath, etc. A standard Watts-type bath containing about 50 to about g.p.l. of nickel, e.g., about 80 g.p.l. of nickel, about 10 to about 20 g.p.l. of chloride ion, e.g., about 18 g.p.l. of chloride ion, about 80 to about 170 g.p.l. of sulfateion, e.g., about g.p.l. of sulfate ion, about 20 to about 45 g.p.l. of boric acid, e.g., about 30 g.p.l. of boric acid, having a pH of about 1.0 to about 4.5, e.g., about pH 4, a temperature of about F. to about F., e.g., 140 F., employed at a cathode current density of about 20 to about 40 a.s.f. is

i a satisfactory nickel plating bath. The nickel deposit may be applied by means of a standard electroless nickel plating technique employing, for example, a bath containing about 30 g.p.l. nickel chloride (NiCl .NH O), about 10 g.p.l. sodium hypophosphite (NaH PO .H O), about 50 g.p.l. sodium hydroxyacetate (NaC H O having a pH of about 6 to 7 and a temperature of about F. to 200 F. A standard aqueous chromium plating bath containing about 250 to about 400 g.p.l. of chromic acid and about 2.5 to about 4.0 g.p.l. of sulfuric acid may be employed for the purpose of producing chromium coatings. A standard aqueous cadmium plating bath containing 22.5 to about 40 g.p.l. of cadmium oxide and 85 to 135 g.p.l. of sodium cyanide may be employed to produce cadimum coatings.

In order to give those skilled in the art a better understanding of the present invention, the following illustrative example is given.

EXAMPLE A series of ten sheet panels made of 18% nickel maraging steel of the'250 grade and containing about 18% nickel, about 8% cobalt, about molybdenum, about 0.4% titanium, about 0.02% carbon and the balance essentially iron was prepared. The panels were fully aged by heating at about 900 F. for about 3 hours and were then polished, shot peened and demagnetized. The panels Were cleaned in an alkaline bath containing about 60 g.p.l. of sodium carbonate and having a temperature of 180 F. by a cathodic treatment for about 1 minute at 75 a.s.f. The panels were then pumice scrubbed, rinsed and given a second cathodic cleaning under the same conditions as described hereinbefore, after which they were given an anodic treatment in the same bath at 75 a.s.f. for about 15 seconds. The panels were then water rinsed and given an anodic treatment in an aqueous bath containing about 25% by weight of concentrated sulfuric acid, using an anodic current density of 200 a.s.f. for 2 minutes at room temperature. The panels were again water rinsed and dipped for 1 minute at room temperature in a Water solution containing about 45 g.p.l. of chromic acid and about 20 milliliters per liter (about 36.8 grams per liter) of sulfuric acid. The panels were then given hot and cold water rinses and were transferred to an aqueous acid nickel chloride bath containing about 240 g.p.l. of nickel chloride (NiCl .6H O) and about 38 g.p.l. of hydrochloric acid at room temperature. After a one minute immersion in the bath, each panel was plated in the bath for about one minute at a cathode current density of about 30 a.s.f. and was then transferred without rinsing to a Watts-type nickel plating bath containing about 80 g.p.l. of nickel, about 18 g.p.l. of chloride ion, about 110 g.p.l. of sulfate ion, about 30 g.p.l. of boric acid, having a pH of 4 and a temperature of 140 F. The panels were plated at a cathode current density of about 30 a.s.f. to deposit 50 mils of nickel thereon. The panels were then machined into adhesion specimens as described in the article by B. B. Knapp appearing in Metal Finishing, December 1949, at pages 40 to 46, inclusive. The results of the adhesion tests are set forth in the following table:

Table Specimen No.: Bond strength, psi.

Examination of the adhesion test specimens demonstrated that, in most cases, failure was due to rupture of the nickel coating with the nickel remaining adherent to the basis metal. The average bond strength of 69,530 pounds per square inch (p.s.i.) determined for the series of panels was, accordingly, regarded as being related to the inherent strength of the nickel deposit. In contrast to the foregoing, similar panels subjected to similar pretreatment steps only through the anodic sulfuric acid treatment and then nickel plated in a similar manner displayed bond strengths in the same test of only 24,000 p.s.i.

It was found that further test panels plated in accordance with the special method of the invention described in the foregoing example displayed strong adhesion of nickel to the basis metal when subjected to grinding and filing tests. In addition, further test panels prepared in accordance with the invention as described in the foregoing example were subjected to baking in air at temperatures of 400 F., 600 F. and 1500 F. for periods of from 4 to 24 hours without the occurrence of blistering or other signs of poor adhesion. In additional tests, cylindrical specimens of extruded 18% nickel 250 grade maraging steel inches long and having an outside diameter of about 2%. inches were subjected in the fully maraged and shot peened condition to the preparation method within the invention as described in the foregoing example. These cylindrical specimens were plated with 1 mil of nickel followed by 4 mils of chromium. The chromium plated specimens were then baked for 24 hours at 600 F. Adhesion of the electrodeposited coating was good. In a still further test, the cylindrical portion of a fully hardened and shot peened 18% nickel 250 grade maraging steel landing gear strut about 25 inches long and having an outside diameter of 3 /4 inches was prepared for plating in accordance with the method of the invention as described in the aforementioned example and was plated with 1.5 mils of Watts-type nickel and 8 mils of standard chromium. The strut was also baked for 24 hours at 600 F. and it was found that adhesion of the coating was good.

It Will be appreciated that metal coatings are applied to maraging steel parts employed in critical service conditions as the last stage in processing of the parts. Accordingly, it is essential that the plater be in a position to guarantee that the metal coatings are, practically speaking, 100% acceptable. Any lesser degree of precision can result in total rejection of completed parts at the most expensive stage in manufacture. The special three-stage surface conditioning process enables the plater to prepare metal-coated maraging steel parts with the required acceptability. Experience has indicated that the three sequential surface-conditioning process steps described hereinbefore, i.e., the anodic sulfuric acid treatment, the dip in sulfuric acid-chromic acid solution, and the nickel strike in acid nickel chloride solution should be employed to provide consistently good adherence of a nickel coating to 18% nickel maraging steel. Omission of any of the surface conditioning steps can produce a marked reduction in acceptability of the metal coating applied. The entire treating operation in accordance with the invention can be conducted in about 5 minutes, substantial practical advantage.

It is important to maintain conditions in each of the preparation steps as set forth hereinbefore. Thus, in the anodic sulfuric acid treatment, the sulfuric acid content of the bath is maintained in an effective concentration of at least about 250 g.p.l. but does not exceed about 375 g.p.l. in order to avoid excessive etching of the steel surface. Anodic current density of at least about 200 to about 250 a.s.f. is employed because at values beneath this range dissolution of the steel article occurs and at values exceeding this range film formation on the article occurs rendering the article passive. In the sulfuric acidchromic acid dip, the sulfuric acid concentration is maintained in the effective range of at least 1 g.p.l. but does not exceed about 90 g.p.l. to avoid active dissolution of the steel article and formation of a heavy black film on the surface, and the chromic acid (CrO concentration is maintained in the effective range of about 10 to about 500 g.p.l. In the nickel strike bath, nickel chloride (NiCl .6H O) is maintained between about 200 and about 300 g.p.l. and the hydrochloric acid concentration is maintained between about 30 and about 40 g.p.l. It is not necessary to heat any of the solutions above room temperature, although temperatures up to about F. can be employed in each solution. Of course, in common with all plating, operations, the metal surface to be coated in accordance with the inventionmust be thoroughly cleaned to remove all surface dirt, grease, mill scale, heat-treating scale, etc., before the surface conditioning operations contemplated in accordance with the invention are conducted.

Maraging steel compositions which may be treated in accordance with the present invention include the 18% nickel maraging steel containing about 17% to about 19% nickel, about 7% to about 9.5% cobalt, about 4.6% to about 5.2% molybdenum, about 0.3% to about 0.7% titanium, about 0.01% to about 0.03% carbon and the balance essentially iron. The steels identified in the field as the 18% nickel 250 grade maraging steels have a minimum yield strength in the fully aged condition of at least about 250,000 p.s.i. and contain about 17% to 19% nickel, about 7% to about 8.5% cobalt, about 4.6% to about 5.1% molybdenum, about 0.3% to about 0.5% titanium, about 0.01% to about 0.03% carbon, and the balance essentially iron. The 18% nickel maraging steel compositions identified in the field as the 300 grade maraging steel, i.e., having a minimum yield strength in the maraged condition of at least about 300,000 p.s.i., contain about 18% to 19% nickel, about 8.5% to about 9.5% cobalt, about 4.7% to about 5.2% molybdenum, about 0.5% to about 0.7% titanium, about 0.01% to about 0.03% carbon, and the balance essentially iron. Such steel compositions are described in the Decker, Goldman and Eash US. Patent No. 3,093,519. Other steel compositions which may be treated with the special procedure provided in accordance with the invention include the 20% nickel and 25% nickel maraging steels described in the Bieber US. Patent No. 3,093,518 and the 12% nickel maraging steels described in US. patent applications Ser. No. 286,365 and Ser. No. 382,309, now US. Patents No. 3,262,823 and 3,262,777 respectively.

It will be appreciated that chromium plated maraging steels generally require a baking treatment to remove hydrogen introduced during the plating operation. Baking is usually accomplished at temperatures of about 450 F. to about 600 F. for baking periods up to about 24 hours and the electrodeposited coating must withstand this treatment without damage. If desired, the steel articles can be plated in the softened condition with nickel and/ or chromium and can be subjected to a maraging heat treatment in the as-plated condition since such heat treatments seldom exceed about 900 F.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention' as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. In the process for plating a maraging steel article, the improvement comprising conditioning the surface of said article to be plated by sequentially subjecting said surface to anodic treatment for at least about 1 minute in an aqueous solution containing about 250 to about 375 grams per liter of sulfuric acid at a current density of about 200 to about 250 a.s.f., subjecting said surface to a dip in an aqueous solution containing about 1 to about 90 grams per liter of sulfuric acid and about to about 500 grams per liter of chromic acid, applying a nickel strike coating to the thus-treated surface by immersing said surface for at least about 1 minute without the use of applied current in an aqueous bath containing about 200 to about 300 grams per liter of nickel chloride and about 30 to about 40 grams per liter of hydrochloric acid and then applying current to said surface as cathode in said bath at a cathodic current density of about 10 to about 50 a.s.f. for at least about 1 minute and thereafter applying a metal coating to the thus-prepared surface, whereby good adherence of said metal coating to said surface is obtained.

2. In the process for plating a maraging steel article, the improvement comprising conditioning the surface of said article to be plated by sequentially subjecting said surface to anodic treatment for at least about 1 minute in an aqueous solution containing about 250 to about 350 grams per liter of sulfuric acid at a current density of about 200 to about 250 a.s.f., immersing said surface for about 1 minute to about 3 minutes in an aqueous solution containing about 40 to about 60 grams per liter of chromic acid and about 35 to about 55 grams per liter of sulfuric acid, applying a nickel strike coating to said surface by immersing said surface for about 1 minute up to about 5 minutes without the use of applied current in an aqueous bath containing about 200 to about 300 grams per liter of nickel chloride and about 30 to about 40 grams per liter of hydrochloric acid and thereafter electrodepositing nickel upon said surface as cathode in said bath for about 1 to about 6 minutes at a cathodic current density of about 10 to about 50 a.s.f., and then applying a metal coating from the group consisting of nickel, cadmium and chromium to the thus-prepared metal surface, whereby good adherence of said metal coating to said surface is obtained.

3. In the process for plating .a maraging steel article, the improvement comprising conditioning the surface of said article to be plated by sequentially subjecting said surface to anodic treatment for about 2 minutes in an aqueous sulfuric acid solution containing about 250 to about 350 grams per liter of sulfuric acid at a current density of about 200 to about 250 a.s.f., immersing said surface for about 1 minute in an aqueous solution containing about 40 to about 60 grams per liter of chromic acid and about 35 to about 55 grams per liter of sulfuric acid, then immersing said surface for about 1 minute without the use of applied current in an aqueous nickel chloride strike solution bath containing about 240 grams per liter of nickel chloride and about 30 to about 40 grams per liter of hydrochloric acid and thereafter electrodepositing nickel on said surface as cathode in said bath for about 1 minute at a cathodic current density of about 10 to about 50 a.s.f., and thereafter applying a metal coating to the thus-prepared surface, whereby good adherence of said metal coating to said metal surface is obtained.

4. In the process for plating a maraging steel article, the improvement comprising conditioning the surface of said article to be plated by sequentially subjecting said surface to anodic treatment for about 2 minutes at about 200 a.s.f. in an aqueous sulfuric acid solution containing about 25% by weight of sulfuric acid, immersing said surface for about 1 minute in an aqueous solution containing about 45 grams per liter of chromic acid and about 36.8 grams per liter of sulfuric acid, then immersing said surface in an aqueous acid nickel chloride strike solution bath containing about 240 grams per liter of nickel chloride and about 38 grams per liter of hydrochloric acid for about 1 minute without the use of applied current followed by electrodepositing nickel on said surface as cathode in said bath for about 1 minute at about 30 a.s.f. and then electrodepositing a metal coating from the group consisting of nickel, cadmium and chromium upon the thus-prepared surface, whereby the adherence of said metal coating to said prepared article is improved.

References Cited UNITED STATES PATENTS 1,878,837 9/1932 Francis 204-34 2,285,548 6/1942. Wesley 20434 2,285,549 6/1942 Wesley et al 204-34 2,748,066 5/ 1956 Whitehouse et a1 20434 2,973,307 2/1961 Hahn 204--34 OTHER REFERENCES Metal Industry, Chromium Plating on Steel, pp. 299-300, Electrometallurgy Supplement, May 12, 1944.

JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Examiner. 

1. IN THE PROCESS FOR PLATING A MARAGING STEEL ARTICLE, THE IMPROVEMENT COMPRISING CONDITIONING THE SURFACE OF SAID ARTICLE TO BE PLATED BY SEQUENTIALLY SUBJECTING SAID SURFACE TO ANODIC TREATMENT FOR AT LEAST ABOUT 1 MINUTE IN AN AQUEOUS SOLUTION CONTAINING ABOUT 250 TO ABOUT 375 GRAMS PER LITER OF SULFURIC ACID AT ACURRENT DENSITY OF ABOUT 200 TO ABOUT 250 A.S.F., SUBJECTING SAID SURFACE TO A DIP IN AN AQUEOUS SOLUTION CONTAINING ABOUT 1 TO ABOUT 90 GRAMS PER LITER OF SULFURIC ACID AND ABOUT 10 TO ABOUT 500 GRAMS PER LITER OF CHROMIC ACID, APPLYING A NICKEL STRIKE COATING TO THE THUS-TREATED SURFACE BY IMMERSING SAID SURFACE FOR AT LEAST ABOUT 1 MINUTE WITHOUT THE USE OF APPLIED CURRENT IN AN AQUEOUS BATH CONTAINING ABOUT 200 TO ABOUT 300 GRAMS PER LITER OF NICKEL CHLORIDE AND ABOUT 30 TO ABOUT 40 GRAMS PER LITER OF HYDROCHLORIC ACID AND THEN APPLYING CURRENT TO SAID SURFACE AS CATHODE IN SAID BATH AT A CATHIODIC CURRENT DENSITY OF ABOUT 10 TO ABOUT 50 A.S.F. FOR AT LEAST ABOUT 1 MINUTE AND THEREAFTER APPLYING A METAL COATING TO THE THUS-PREPARED SURFACE, WHEREBY GOOD ADHERENCE OF SAID METAL COATING TO SAID SURFACE IS OBTAINED.
 4. IN THE PROCESS FOR PLATING A MARAGING STEEL ARTICLE, THE IMPROVEMENT COMPRISING CONDITIONING THE SURFACE OF SAID ARTICLE TO BE PLATED BY SEQUENTIALLY SUBJECTING SAID SURFACE TO ANODIC TREATMENT FOR ABOUT 2 MINUTES AT ABOUT 200 A.S.F. IN AN AQUEOUS SULFURIC ACID SOLUTION CONTAINING ABOUT 25% BY WEIGHT OF SULFURIC ACID, IMMERSING SAID SURFACE FOR ABOUT 1 MINUTE IN AN AQUEOUS SOLUTION CONTAINING ABOUT 45 GRAMS PER LITER OF CHROMIC AID AND ABOUT 36.8 GRAMS PER LITER OF SULVURIC ACID, THEN IMMERSING SAID SURFACE IN AN AQUEOUS ACID NICKEL CHLORIDE STRIKE SOLUTION BATH CONTAINING ABOUT 240 GRAMS PER LITER OF NICKEL CHLORIDE AND ABOUT 38 GRAMS PER LITER OF HYDROCHLORIC ACID FOR ABOUT 1 MINUTE WITHOUT THE USE OF APPLIED CURRENT FOLLOWED BY ELECTRODEPOSITING NICKEL ON SAID SURFACE AS CATHODE IN SAID BATH FOR ABOUT 1 MINUTE AT ABOUT 30 A.S.F. AND THEN ELECTRODEPOSITING A METAL COATING FROM THE GROUP CONSISTING OF NICKEL, CADMIUM AND CHROMIUM UPON THE THUS-PREPARED SURFACE, WHEREBY THE ADHERENCE OF SAID METAL COATING TO SAID PREPARED ARTICLE IS IMPROVED. 